1. Field of the Invention
THIS INVENTION relates to hydraulic systems for tensioning fasteners; to nuts to be tensioned thereby; to tools to be used therewith; to washers and to other like type structures and accessories for use therewith.
2. Prior Art
Hydraulic nuts are known. The specification to Australian Patent No. 625495 (AU-B-25403/88), to the present applicant, describes such a nut. These nuts find applications in a variety of fields, for example in the assembly of turbine casings.
Power industry turbine casings are invariably in two halves, joined axially to make an essentially symmetric shell in which the turbine rotor operates. The joint between the casings must be clamped with sufficient force to withstand the massive forces of separation generated by the action of steam under pressure within, and with a high degree of consistency to prevent leakage or distortion of the casing. Steam turbines operate of necessity at high temperatures, so the nature of the material used in the bolts must resist creep (slow relaxation) under such conditions. This rules out using particularly high-strength heat treated alloy fasteners, so the engineering solution taken is that of using studbolts of large cross sectional area at reduced separation (bolt pitch). This reduced separation means that there is very little working room around the bolts. This creates problems in applying the high torque necessary to provide required tensile load in the member. Most turbine manufacturers have, therefore, opted to recommend a practice of heat induced elongation and subsequent shrinkage of the individual studs for casing bolt tensioning. This is an extremely costly method in the amount of time consumed, and it also has detrimental effects on the integrity of the fasteners.
There has been considerable pressure applied by power generation utility companies to the manufacturers of the turbines to improve the methods used in this area. An “outage” of a baseline generator, in a nuclear plant for example, can cost a considerable sum (in excess of four million dollars) per day. There are economic reasons to improve performance. The utility companies are of the opinion that permanently installed hydraulic fasteners are the ideal method of force application for these studbolts. However, there are design changes necessary to best fit such equipment requiring manufacturers and regulatory body approvals, so they have requested an interim step which would still allow the use of hydraulic tensioning without major design change. Ideally, they would use such equipment as tools which are removed after application, and which do not require replacement of the studbolts themselves.